In the field of graphic arts, in order to improve the reproduction of continuous gradation images with halftone dot images or the reproduction of line images, image formation systems exhibiting ultrahigh-contrast (particularly, .gamma. is 10 or more) photographic characteristics are required.
As methods for obtaining high-contrast photographic characteristics, lithographic development systems utilizing the so-called "infectious development effect" have been used for long. However, they have the disadvantage that the developing solutions are unstable and hard to use. In contrast, as methods using more stable developing solutions, there are methods described in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,221,857, 4,332,878, 4,634,661, 4,618,574, 4,269,922, 5,650,746 and 4,681,836.
These image formation systems are systems for processing hydrazine derivative-containing silver halide photographic materials of the surface latent image type with stable MQ developing solutions (in which hydroquinone is used in combination with a p-aminophenol) or PQ developing solutions (in which hydroquinone is used in combination with a 1-phenyl-3-pyrazolidone compound) having a pH of 11.0 to 12.3 to obtain ultrahigh-contrast negative images in which .gamma. exceeds 10. According to these methods, ultrahigh-contrast and high-speed photographic characteristics can be obtained, and sulfites can be added in high concentration to the developing solutions. Therefore, the stability against air oxidation of the developing solutions is far improved, compared with the conventional lithographic developing solutions.
U.S. Pat. No. 4,269,929 (JP-A-61-267759 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), U.S. Pat. No. 4,737,452 (JP-A-60-179734), U.S. Pat. No. 5,104,769 and U.S. Pat. No. 4,798,780, JP-A-1-179939, JP-A-1-179940, U.S. Pat. Nos. 4,998,604 and 4,994,365, and JP-A-8-272023 disclose methods of using high-active hydrazine nucleating agents and nucleating accelerators for obtaining ultrahigh-contrast images by use of stable developing solutions having a pH of less than 11.0. It is also disclosed that silver halide emulsions high in silver chloride content and chemically sensitized have high nucleating activity. However, the use of the high-active hydrazine nucleating agents and nucleating accelerators as described above or high-active emulsions raises the problem of uneven density at the time of an output of a definite halftone dot area, which is called processing unevenness.
On the other hand, it is known that enediols such as ascorbic acid function as developing agents, and attention is given to them as developing agents having no ecological and toxicological problems. For example, in U.S. Pat. Nos. 2,688,549 and 3,826,654, it is described that image formation is possible under high alkaline conditions of pH 12 or higher. However, high-contrast images can not be obtained by these image formation methods.
Some attempts have been made to increase the contrast in development systems using ascorbic acid. For example, Zwicky describes that the use of ascorbic acid as a sole developing agent causes a kind of lithographic effect to appears (J. Phot. Sci., 27, 185 (1979)). However, this system was considerably low in the contrast, compared with hydroquinone development systems. Further, U.S. Pat. No. T896,022 and JP-B-49-46939 (the term "JP-B" as used herein means an "examined Japanese patent publication") disclose a system in which ascorbic acid is used in combination with a bis-quaternary ammonium salt. This system has the development accelerating effect, but scarcely shows the contrast increasing effect. JP-A-3-249756 and JP-A-4-32838 also describe the effect of using ascorbic acid in combination with a quaternary salt. However, the contrast of images obtained is not sufficient. Furthermore, JP-A-5-88306 describes that high contrast is obtained by using ascorbic acid as a sole developing agent and keeping the pH at 12.0 or higher. However, this system-also has a problem with respect to the stability of a developing solution because of its high pH.
There is an example in which a particular developing solution containing ascorbic acid and a hydrazine derivative as main components is used to obtain a development system high in sensitivity and low in stain and fog (U.S. Pat. No. 3,730,727). However, no reference is made to contrast enhancement.
It is known that photographic materials containing hydrazine are processed with ascorbic acid developing solutions, and disclosed in U.S. Pat. No. 5,236,816 and WO 93/11456. However, both are insufficient in terms of the contrast. In the latter, the contrast is enhanced by adding an amine to a developing solution, but this is environmentally unfavorable. A developing method has therefore been desired in which toxicologically preferable ascorbic acid is used as a developing agent to obtain a high-contrast image. Previously, Metol or phenidone compounds have been used as auxiliary developing agents for the ascorbic acid developing solutions. Also in the above-mentioned patents, either of these auxiliary developing agents is used.
Further, a high-contrast photographic material containing silver halide grains spectrally sensitized with a non-desorptive sensitizing dye, silver halide grains not spectrally sensitized, and a hydrazine derivative is described in the claim of British Patent Publication (Laid-Open) 9,407,599. The light-sensitive grains spectrally sensitized and the light-insensitive grains not spectrally sensitized contribute to silver images formed, by imagewise exposure and development, because of the presence of the hydrazine derivative. However, this photographic material has the disadvantage that, when a developing solution is fatigued or the replenishment rate is low, the development of the light-insensitive grains by the presence of the hydrazine derivative becomes difficult to take place, resulting in reduced density of the silver images, and is not sufficient from the viewpoint of processing stability in running.
The system for forming ultrahigh-contrast images using hydrazine derivatives is a system using dihydroxybenzene compounds such as hydroquinone as developing agents, as described above, and have some disadvantages from the ecological and toxicological viewpoints. For example, hydroquinone is an unfavorable component because of its allergy inducing effect. In this image formation system, amines as described in U.S. Pat. No. 4,975,354 are used simultaneously, and they are unfavorable in terms of toxicity and volatility.